A photoplethysmography (PPG) apparatus measures changes in the volume of an external object in an optical way. In medical applications, these changes in volume usually are changes of the amount of blood or air in an organ or other body part of a subject and can therefore be used to monitor vital-sign information of the subject. Vital-sign information of a subject comprises for instance information on respiratory rate, pulse rate, blood pressure, or blood oxygen saturation of a person.
In WO 2014/136027 A1, also published under U.S. 2014/0253709A1, a system and method for determining vital-sign information of a subject is disclosed. The subject is illuminated with radiation, and radiation reflected from the subject is received. In a first phase, a region of interest is located. In a second phase, the illumination is controlled to locally illuminate the located region of interest. Finally, vital-sign information of the subject is determined from the radiation reflected from the region of interest and detected in said second phase.